The reading is from St. Paul's Letter to the Galatians 2:11-16

Brethren, when Cephas came to Antioch I opposed him to his face, because he stood condemned. For before certain men came from James, he ate with the Gentiles; but when they came he drew back and separated himself, fearing the circumcision party. And with him the rest of the Jews acted insincerely, so that even Barnabas was carried away by their insincerity. But when I saw that they were not straightforward about the truth of the gospel, I said to Cephas before them all, "if you, though a Jew, live like a Gentile and not like a Jew, how can you compel the Gentiles to live like Jews?" We ourselves, who are Jews by birth and not Gentile sinners, yet who know that a man is not justified by works of the law but through faith in Jesus Christ, even we have believed in Christ Jesus, in order to be justified by faith in Christ, and not by works of the law, because by works of the law shall no one be justified.